Electrical heating devices



Jan. 14, 1964 K. PARKER 3,113,042

ELECTRICAL HEATING DEVICES Filed Oct. 26; 1961 LESLIE KEARTON PARKERUnited States Patent 3,118,042 ELECTRICAL HEATING DEVICES Leslie KeartonParker, Beaconsfield, England, assignor to The Perkin-Elmer CorporationFiled 0ct. 26, 1961, Ser. No. 147,840 Claims priority, application GreatBritain Nov. 16, 1960 5 Claims. (Cl. 219-19) This invention relates toelectrical heating apparatus and, more particularly, to electricalheating devices adapted for use with scientific instruments.

Electrical heating devices are known which are suitable for use inmaintaining predetermined temperatures. However, such devices are notsatisfactory for use with certain kinds of scientific apparatus sincethe heating, produced at intervals to maintain the temperature, isaccompanied by magnetic fields which influence the apparatus.

It is, therefore, a primary object of the present invention to provideimproved heating apparatus. Other objects are to provide such heatingapparatus which is highly accurate; which is easily controlled; whichproduces a minimal magnetic field; and which is particularly welladapted for use with a nuclear magnetic resonance device.

According to the present invention, an electrical heating device isprovided having one or more electrical heating elements made up ofconductors arranged in pairs. The conductors of each pair are closelyadjacent one another and conduct current in opposite directions, thusproducing heat but negligible magnetic field. The device also has one ormore electrically conducting elements which respond to changes oftemperature produced by the heating. These elements are also made up ofconductors arranged in pairs, the conductors of a pair being closelyadjacent one another and arranged to conduct current in oppositedirections. In use, the temperature-responsive elements are connectedthrough a control circuit, for example, a standard bridge circuit, so asto control the application of current to the heating elements. Currentis then applied when the temperature detected by thetemperature-responsive elements falls below a predetermined level.Because of the arrangement of the parts making up the elements, nosignificant magnetic field is produced by the device.

Preferably each of the conducting elements is in the form of aresistance wire, the resistance of which changes with a change oftemperature. Alternatively, however, there may be a number of suchconducting elements making up one or more thermocouples.

Preferably, the heating elements are in heat-transfer relationship with,but are electrically insulated from, a heat-conducting screen. Thescreen supports the temperature-responsive elements so that the latterrespond to temperature changes produced by the heating. The heatingelements may be embedded in the screen, although in a convenientarrangement the heating elements are mounted on one of its faces. Thetemperature-responsive elements are mounted on the opposite face of thescreen. In use, the screen is mounted with the temperature-responsiveelements adjacent the apparatus whose temperature is to be controlled.The screen itself serves to spread out in a more uniform manner the heatproduced by the heater element or elements and thus enables theapparatus to be heated more uniformly. The screen may be in the form ofa continuous flat sheet, for example, of aluminum.

Preferably the conductors making up the heating elements are in the formof strips. The strips of a pair are substantially identical and aresuperimposed with their larger surfaces parallel to the larger surfacesof the screen. In this way a good heat-transfer relationship may beobtained between the elements and the screen, while the magnetic fieldsproduced may be reduced to a desirably low level.

Preferably the heating elements extend over substantially the whole ofthe first face of the screen. In this way, substantially the whole ofthe screen provides a heating surface. Preferably also thetemperature-responsive elements .are in the form of resistance wiresextending over substantially the whole of the second face of the screen.The Wires maythen be arranged to respond to an average temperature. Inone convenient arrangement for example, the heating elements on the onehand, and the wires on the other hand, extend for the greater part oftheir length parallel to one another in the device. The resistance wiresare disposed, however, not along the area of maximum heating (that is tosay the area directly opposite the heating elements) nor along the areaof minimum heating (i.e., the area directly opposite the centers of thespaces between the heater elements) but along intermediate areas. Thisis important since the difference between the temperatures at the areasof maximum and minimum heating varies with the ambient temperature,increasing as the ambient temperature falls. Thus, in one particulararrangement, the temperature varies substantially sinusoidally acrossthe screen transverse to the length of the heating elements. Theamplitude of these variations increases with falling ambienttemperature, the maximum temperatures being relatively constant but theminimum temperature decreasing. By locating the resistance wires alongareas intermediate to those of maximum and minimum heating (and thustemperature) it is possible to have these wires respond, irrespective ofthe ambient temperature, to a temperature which is an average of themaximum and minimum temperatures.

Conveniently each heating element and each wire is made up of relativelylong parallel reaches and of relatively short cross reaches whichconnect the long reaches together in series.

A number of devices may be connected together to make up a box structurefor housing the apparatus whose temperature is to be controlled. Thisbox structure may have the heating elements arranged on the outside andthe resistance Wires arranged within and may have four walls, a top, anda bottom, each constructed in accordance with the invention. An openingmay be provided to enable access to be gained to the interior of the boxstructure, and this opening may be closed by a door which itselfconstitutes a heating device in accordance with the invention.

One example of a device constructed in accordance with the inventionwill now be described with reference to the attached drawing wherein:

FIGURE 1 is a plan view of a box structure opened out and laid fiat, butwith one Wall missing and two others only partially shown; and

FIGURE 2 is a cross-section along the line IIII of FIGURE 1.

The box structure comprises four side walls three of which are shown as1a, 1b, and la, a top 2, and a bottom 3, each of these comprising anelectrical heating device in accordance with the invention. Thesedevices are basically very similar to one another and only one, the wallIn, will therefore be described in detail. The wall 1a includes aheat-conducting screen in the form of a rectangular sheet 6 of aluminumMs" thick. Mounted on the outer face of this sheet 6 are two identicalheating elements, in the form of thin aluminum strips, the outer strip 7being superimposed exactly in register on the inner strip, with theirlarge surfaces parallel to the larger surfaces of the sheet 6. The twostrips are insulated from one another and from the sheet 6 by thinlayers of electrical insulation, conveniently of manila paper. Thismaterial permits heat to be transferred from the strips to the plate 6.

Each strip is made up of relatively long parallel reaches 9 and ofrelatively short cross reaches 10 which connect the long reaches 9together in series. The long and short reaches are integral with oneanother being connected together by bent over portions 11. It will beappreciated that the reaches each comprise an outer and an inner strip.The strips are closely adjacent one another (being separated by a gapof, for example 0.004"). They are also able to conduct current inopposite directions, terminals 1316 being provided on the sheet 6 topermit this to be done. Terminals 13 and 16 are permanently connected tothe ends of the outer strip 7, while terminals 14 and 15 are permanentlyconnected to the ends of the lower strip. Thus, the passage of currentthrough the two strips enables heat to be produced with negligiblemagnetic field, this heat passing through the sheet 6 which serves tospread it out in a more uniform manner.

Mounted on the inner face of the sheet 6 are a pair of resistance wires18 which change resistance in response to changes of temperatureproduced by the heating and which are represented in FIGURE 1 by doubledotted lines 17. These two wires 18 are parallel to the strips and areelectrically insulated from the sheet 6. Wires 18 are also made up ofrelatively long parallel reaches 19 and relatively short cross reaches20 which connect the long reaches 19 together in series. It can be seenthat these reaches are arranged in pairs, and that the reaches of a pairare closely adjacent one another and able to conduct current in oppositedirections. Terminals are provided on the sheet 6 to permit thisapplication of current. In use, these terminals are connected to astandard bridge network which controls the application of current to thestrips.

Despite the good conductivity of the sheet 6, the temperature at itsinner face is not the same at all points and the wires 18 are,therefore, arranged so as to be subjected to the average temperatureirrespective of the ambient temperature. It is for this reason thatthese wires 18 are disposed not along the area of maximum heating noralong the area of minimum heating but along the intermediate area.

The other walls are the same as the wall In, the resistance wires notbeing shown in the drawing.

The top 2 and the bottom 3 are also similar to the wall In (althoughagain the resistance wires which are arranged as before with respect tothe heater strips, are omitted) but differ in that each is provided withtwo pairs of strips rather than one pair, and a central opening isprovided. The opening 21 in the top 2 is to enable access to be gainedto apparatus mounted within the box structure, while the opening 22 inthe bottom 3 permits air to be circulated inside the box structure bymeans of an electric fan.

The illustrated structure is suitable for housing the large permanentmagnet of a nuclear magnetic resonance apparatus, and is able to keepthe magnet at a substantially uniform temperature to within amillidegree at a temperature of the order of 10-20 C. above the ambienttemperature.

It is to be understood that the apparatus of this invention is subjectto a number of variations and modifications without departing from thespirit thereof. The foregoing description is illustrative only, thescope of the invention is to be construed as limited only by the scopeof the following claims.

I claim:

1. A heater assembly which comprises an enclosure defined by 6 walls,each of said walls comprising a heat-conductive screen; firstelectrically conductive heating member means mounted on the outersurface of said screen and adapted to carry electrical current in afirst direction; second electrically conductive heating member meansclosely adjacent said first heating member means but electricallyinsulated therefrom and adapted to carry electrical current in a seconddirection; first temperature sensing conductor means mounted on theinner surface of said screen and adapted to carry electrical current ina first direction; second temperature sensing conductor means closelyadjacent said first conductor means but electrically insulated therefromand adapted to carry electrical current in a second direction, all ofsaid first and second heating member means and said first and secondconductor means being in heat exchanging relationship with said screenbut electrically insulated therefrom and an opening in one wall topermit air circulation in side the enclosure.

2. The apparatus of claim 1 wherein said screen is a flat metallicsheet, said first and second heating member means are superposedelongated ribbon-like strips, and the longitudinal axes of all of saidheating member means and said conductor means are parallel.

3. Electrical heating apparatus comprising a flat heat conductivescreen, a pair of superposed, coinciding electrically conductive heatingmembers positioned on one side of said screen, and being electricallyinsulated from each other and from said screen, said heating membersbeing adapted to carry current in opposite directions thereby minimizingresulting inductive effects, first temperature sensing conductor meansadapted to carry current in a first direction, a second temperaturesensing conductor means adapted to carry current in a direction oppositeto said first direction, said first and second temperature sensingconducting means being disposed adjacent and insulated from each otherand from the screen, the said first and second temperature conductingmeans being disposed on the opposite side of the flat screen from saidheating members.

4. Apparatus as defined in claim 3 in which said fiat screen is ametallic sheet.

5. Apparatus as defined in claim 3 wherein the longitudinal axes of saidfirst and second conductor means are parallel to the longitudinal axesof said pair of heating members.

References Cited in the file of this patent UNITED STATES PATENTS897,782 Reilley Sept. 1, 1908 1,170,811 Hay Feb. 8, 1916 1,972,720Tarpley et a1. Sept. 4, 1934 2,487,695 Cloud Nov. 8, 1949 2,505,117Hoffman Apr. 25, 1950 2,568,600 Wirk Sept. 18, 1951 2,717,945 Dresios etal Sept. 13, 1955 2,855,495 Grant Oct. 7, 1958 3,007,023 Johnston et al.Oct. 31, 1961 3,010,007 Theodore et al. Nov. 21, 1961 FOREIGN PATENTS570,026 Great Britain June 19, 1945

3. ELECTRICAL HEATING APPARATUS COMPRISING A FLAT HEAT CONDUCTIVESCREEN, A PAIR OF SUPERPOSED, COINCIDING ELECTRICALLY CONDUCTIVE HEATINGMEMBERS POSITIONED ON ONE SIDE OF SAID SCREEN, AND BEING ELECTRICALLYINSULATED FROM EACH OTHER AND FROM SAID SCREEN, SAID HEATING MEMBERSBEING ADAPTED TO CARRY CURRENT IN OPPOSITE DIRECTIONS THEREBY MINIMIZINGRESULTING INDUCTIVE EFFECTS, FIRST TEMPERATURE SENSING CONDUCTOR MEANSADAPTED TO CARRY CURRENT IN A FIRST DIRECTION, A SECOND TEMPERATURESENSING CONDUCTOR MEANS ADAPTED TO CARRY CURRENT IN A DIRECTION OPPOSITETO SAID FIRST DIRECTION, SAID FIRST AND SECOND TEMPERATURE SENSINGCONDUCTING MEANS BEING DISPOSED ADJACENT AND INSULATED FROM EACH OTHERAND FROM THE SCREEN, THE SAID FIRST AND SECOND TEMPERATURE CONDUCTINGMEANS BEING DISPOSED ON THE OPPOSITE SIDE OF THE FLAT SCREEN FROM SAIDHEATING MEMBERS.